Changing main domain name

[digiplay]

We are running Ubuntu 8.10 with ISPconfig 3.0.

I used a valid FQDN setting up the new system, but discovered that I had some weird problems with routing SMTP in that the base domain name was used elsewhere. I want to change the domain name to avoid this problem.

Example:

Currently: webserver.mydomain.tld

Change to: webserver.myotherdomain.tld

What do I need to do? I'm new at this and didn't want to proceed with out knowing what possible things might go crazy and what can be done to avoid this. As you can see, I'm leaving the first part the same and will leave the machine name the same.

Thanks.